.TH DJPEG 1 "17 June 2024"
.SH NAME
djpeg \- decompress a JPEG file to an image file
.SH SYNOPSIS
.B djpeg
[
.I options
]
[
.I filename
]
.LP
.SH DESCRIPTION
.LP
.B djpeg
decompresses the named JPEG file, or the standard input if no file is named,
and produces an image file on the standard output.  PBMPLUS (PPM/PGM), BMP,
GIF, or Targa output format can be selected.
.SH OPTIONS
All switch names may be abbreviated; for example,
.B \-grayscale
may be written
.B \-gray
or
.BR \-gr .
Most of the "basic" switches can be abbreviated to as little as one letter.
Upper and lower case are equivalent (thus
.B \-BMP
is the same as
.BR \-bmp ).
British spellings are also accepted (e.g.
.BR \-greyscale ),
though for brevity these are not mentioned below.
.PP
The basic switches are:
.TP
.BI \-colors " N"
Reduce image to at most N colors [legacy feature].  This reduces the number of
colors used in the output image so that it can be stored in a colormapped file
format.  This feature cannot be used when decompressing lossless JPEG images.
.TP
.BI \-quantize " N"
Same as
.BR \-colors .
.B \-colors
is the recommended name.
.B \-quantize
is provided only for backward compatibility.
.TP
.B \-fast
Select recommended processing options for low-quality output [legacy feature].
(The default options are chosen for highest-quality output.)  Currently, this
is equivalent to \fB\-dct fast \-nosmooth \-onepass \-dither ordered\fR.  On
modern CPUs, these settings have little or no performance benefit and are
retained solely for backward compatibility.
.TP
.B \-grayscale
Force grayscale output even if JPEG file is full-color.  This feature cannot be
used when decompressing full-color lossless JPEG images.
.TP
.B \-rgb
Force RGB output even if JPEG file is grayscale.  This feature cannot be used
when decompressing grayscale lossless JPEG images.
.TP
.BI \-scale " M/N"
Scale the output image by a factor M/N.  Currently the scale factor must be
M/8, where M is an integer between 1 and 16 inclusive, or any reduced fraction
thereof (such as 1/2, 3/4, etc.)  Scaling is handy if the image is larger than
your screen.  This feature cannot be used when decompressing lossless JPEG
images.
.TP
.B \-bmp
Select BMP output format (Windows flavor).  8-bit colormapped format is
emitted if
.B \-colors
or
.B \-grayscale
is specified, or if the JPEG file is grayscale; otherwise, 24-bit full-color
format is emitted.  This format can only be used when decompressing
8-bit-per-sample JPEG images.
.TP
.B \-gif
Select GIF output format (LZW-compressed) [legacy feature].  Since GIF does not
support more than 256 colors,
.B \-colors 256
is assumed (unless you specify a smaller number of colors).  If you specify
.BR \-fast,
the default number of colors is 216.  This format can only be used when
decompressing 8-bit-per-sample or 12-bit-per-sample lossy JPEG images.
.TP
.B \-gif0
Select GIF output format (uncompressed) [legacy feature].  Since GIF does not
support more than 256 colors,
.B \-colors 256
is assumed (unless you specify a smaller number of colors).  If you specify
.BR \-fast,
the default number of colors is 216.  This format can only be used when
decompressing 8-bit-per-sample or 12-bit-per-sample lossy JPEG images.
.TP
.B \-os2
Select BMP output format (OS/2 1.x flavor) [legacy feature].  8-bit colormapped
format is emitted if
.B \-colors
or
.B \-grayscale
is specified, or if the JPEG file is grayscale; otherwise, 24-bit full-color
format is emitted.  This format can only be used when decompressing
8-bit-per-sample JPEG images.
.TP
.B \-pnm
Select PBMPLUS (PPM/PGM) output format (this is the default format).
PGM is emitted if the JPEG file is grayscale or if
.B \-grayscale
is specified; otherwise PPM is emitted.
.TP
.B \-targa
Select Targa output format [legacy feature].  Grayscale format is emitted if
the JPEG file is grayscale or if
.B \-grayscale
is specified; otherwise, colormapped format is emitted if
.B \-colors
is specified; otherwise, 24-bit full-color format is emitted.  This format can
only be used when decompressing 8-bit-per-sample JPEG images.
.PP
Switches for advanced users:
.TP
.B \-dct int
Use accurate integer DCT method (default).
.TP
.B \-dct fast
Use less accurate integer DCT method [legacy feature].
When the Independent JPEG Group's software was first released in 1991, the
decompression time for a 1-megapixel JPEG image on a mainstream PC was measured
in minutes.  Thus, the \fBfast\fR integer DCT algorithm provided noticeable
performance benefits.  On modern CPUs running libjpeg-turbo, however, the
decompression time for a 1-megapixel JPEG image is measured in milliseconds,
and thus the performance benefits of the \fBfast\fR algorithm are much less
noticeable.  On modern x86/x86-64 CPUs that support AVX2 instructions, the
\fBfast\fR and \fBint\fR methods have similar performance.  On other types of
CPUs, the \fBfast\fR method is generally about 5-15% faster than the \fBint\fR
method.

If the JPEG image was compressed using a quality level of 85 or below, then
there should be little or no perceptible quality difference between the two
algorithms.  When decompressing images that were compressed using quality
levels above 85, however, the difference between the \fBfast\fR and \fBint\fR
methods becomes more pronounced.  With images compressed using quality=97, for
instance, the \fBfast\fR method incurs generally about a 4-6 dB loss in PSNR
relative to the \fBint\fR method, but this can be larger for some images.  If
you can avoid it, do not use the \fBfast\fR method when decompressing images
that were compressed using quality levels above 97.  The algorithm often
degenerates for such images and can actually produce a more lossy output image
than if the JPEG image had been compressed using lower quality levels.
.TP
.B \-dct float
Use floating-point DCT method [legacy feature].
The \fBfloat\fR method does not produce significantly more accurate results
than the \fBint\fR method, and it is much slower.  The \fBfloat\fR method may
also give different results on different machines due to varying roundoff
behavior, whereas the integer methods should give the same results on all
machines.
.TP
.B \-dither fs
Use Floyd-Steinberg dithering when quantizing colors [legacy feature].
.TP
.B \-dither ordered
Use ordered dithering when quantizing colors [legacy feature].
.TP
.B \-dither none
Do not use dithering when quantizing colors [legacy feature].  By default,
Floyd-Steinberg dithering is applied when quantizing colors.  This is slower
but usually produces the best results.  Ordered dithering is a compromise
between speed and quality.  No dithering is faster but usually looks awful.
Note that these switches have no effect unless color quantization is being
done.  Ordered dithering is only available in
.B \-onepass
mode.
.TP
.BI \-icc " file"
Extract ICC color management profile to the specified file.
.TP
.BI \-map " file"
Quantize to the colors used in the specified image file [legacy feature].  This
is useful for producing multiple files with identical color maps, or for
forcing a predefined set of colors to be used.  The
.I file
must be a GIF or PPM file. This option overrides
.B \-colors
and
.BR \-onepass .
.TP
.B \-nosmooth
Use a faster, lower-quality upsampling routine.
.TP
.B \-onepass
Use one-pass instead of two-pass color quantization [legacy feature].  The
one-pass method needs less memory, but it produces a lower-quality image.
.B \-onepass
is ignored unless you also specify
.B \-colors
.IR N .
Also, the one-pass method is always used for grayscale output.  (The two-pass
method has no improvement in that case.)
.TP
.BI \-maxmemory " N"
Set limit for amount of memory to use in processing large images.  Value is
in thousands of bytes, or millions of bytes if "M" is attached to the
number.  For example,
.B \-max 4m
selects 4000000 bytes.  If more space is needed, an error will occur.
.TP
.BI \-maxscans " N"
Abort if the JPEG image contains more than
.I N
scans.  This feature demonstrates a method by which applications can guard
against denial-of-service attacks instigated by specially-crafted malformed
JPEG images containing numerous scans with missing image data or image data
consisting only of "EOB runs" (a feature of progressive JPEG images that allows
potentially hundreds of thousands of adjoining zero-value pixels to be
represented using only a few bytes.)  Attempting to decompress such malformed
JPEG images can cause excessive CPU activity, since the decompressor must fully
process each scan (even if the scan is corrupt) before it can proceed to the
next scan.
.TP
.BI \-outfile " name"
Send output image to the named file, not to standard output.
.TP
.BI \-memsrc
Load input file into memory before decompressing.  This feature was implemented
mainly as a way of testing the in-memory source manager (jpeg_mem_src().)
.TP
.BI \-report
Report decompression progress.
.TP
.BI \-skip " Y0,Y1"
Decompress all rows of the JPEG image except those between Y0 and Y1
(inclusive.)  Note that if decompression scaling is being used, then Y0 and Y1
are relative to the scaled image dimensions.
.TP
.BI \-crop " WxH+X+Y"
Decompress only a rectangular subregion of the image, starting at point X,Y
with width W and height H.  If necessary, X will be shifted left to the nearest
iMCU boundary, and the width will be increased accordingly.  Note that if
decompression scaling is being used, then X, Y, W, and H are relative to the
scaled image dimensions.  Currently this option only works with the
PBMPLUS (PPM/PGM), GIF, and Targa output formats.
.TP
.BI \-strict
Treat all warnings as fatal.  This feature also demonstrates a method by which
applications can guard against attacks instigated by specially-crafted
malformed JPEG images.  Enabling this option will cause the decompressor to
abort if the JPEG image contains incomplete or corrupt image data.
.TP
.B \-verbose
Enable debug printout.  More
.BR \-v 's
give more output.  Also, version information is printed at startup.
.TP
.B \-debug
Same as
.BR \-verbose .
.TP
.B \-version
Print version information and exit.
.SH EXAMPLES
.LP
This example decompresses the JPEG file foo.jpg and saves the output in 8-bit
BMP format in foo.bmp:
.IP
.B djpeg \-bmp
.I foo.jpg
.B >
.I foo.bmp
.SH ENVIRONMENT
.TP
.B JPEGMEM
If this environment variable is set, its value is the default memory limit.
The value is specified as described for the
.B \-maxmemory
switch.
.B JPEGMEM
overrides the default value specified when the program was compiled, and
itself is overridden by an explicit
.BR \-maxmemory .
.SH SEE ALSO
.BR cjpeg (1),
.BR jpegtran (1),
.BR rdjpgcom (1),
.BR wrjpgcom (1)
.br
.BR ppm (5),
.BR pgm (5)
.br
Wallace, Gregory K.  "The JPEG Still Picture Compression Standard",
Communications of the ACM, April 1991 (vol. 34, no. 4), pp. 30-44.
.SH AUTHOR
Independent JPEG Group
.PP
This file was modified by The libjpeg-turbo Project to include only information
relevant to libjpeg-turbo, to wordsmith certain sections, and to describe
features not present in libjpeg.
